The present invention relates to an exhaust purification device that purifies exhaust gas with an additive added to the exhaust gas.
Japanese Laid-Open Patent Publication No. 2000-8833 describes a known example of a urea selective catalytic reduction (SCR) device serving as an exhaust purification device that purifies exhaust gas by lowering nitrogen oxide (hereinafter referred to as “NOx”) from the exhaust gas. The urea SCR device uses an adding valve that adds urea water to exhaust gas and a selective reduction catalyst located at a downstream side of the adding valve. In the urea SCR device, the urea water added to exhaust gas is hydrolyzed to ammonia by the heat of the exhaust gas. When the exhaust gas containing the ammonia flows into the selective reduction catalyst, NOx in the exhaust gas is reduced to nitrogen and water by the ammonia serving as an additive.
In order to efficiently lower NOx in a selective reduction catalyst, it is preferred that an additive be sufficiently dispersed when flowing into the selective reduction catalyst. The dispersibility, which indicates the dispersion degree of the additive in exhaust gas, has a tendency to increase as the distance from the location where urea water is added to the selective reduction catalyst becomes longer. However, this enlarges the exhaust purification device. Such a problem is not limited to an exhaust purification device using urea water as an additive and also occurs in an exhaust purification device that purifies exhaust gas with an additive added to the exhaust gas.